It is often desirable to clean or remove coatings from the surfaces of various types of structures and equipment, varying from buildings to industrial devices. Numerous techniques are known for such purposes, ranging from mechanical abrasion techniques to the application of chemicals for cleaning or removing surface coatings such as paint, sealants, lacquers or the like. Hard, durable surfaces, such as granite walls or heavy steel plating may be cleaned or stripped by vigorous abrasive techniques such as sand blasting. More delicate surfaces may require less aggressive treatments to prevent damage to the substrates.
Both commercial airlines and military agencies spend large sums in periodically stripping or abrading paint and other coatings from the exterior surfaces of modern aircraft. These surfaces comprise light weight aluminum or other metal alloys, or composites, which are relatively soft and from which paint or other coatings must be carefully removed to avoid excessive abrasion or chemical damage. Such damage may, in extreme cases, lead to mechanical failure.
Various improved stripping techniques, similar to sand blasting, have been proposed for removing coatings from sensitive metal and composite aircraft or like surfaces. Blasting media useful for such purposes should, preferably, meet the following criteria:
1. They should be relatively non-aggressive (Mohs hardness of about 2.0-3.0);
2. They should be available in various particle size distributions for the treatment of different substrates;
3. They should be free-flowing under high humidity conditions and throughout a broad range of air pressure and media flow rates; and
4. They should be water soluble and non-polluting to facilitate easy separation from the insoluble paints and resins stripped to facilitate waste disposal.
Carr U.S. Pat. No. 4,731,125 granted Mar. 15, 1988 describes the use of plastic media for the blast cleaning of sensitive metal and composite surfaces. Such materials are however, relatively expensive, and their use may impose waste disposal problems.
Sodium bicarbonate has also been proposed as a blasting medium for removing coatings from sensitive substrates such as aircraft parts. Bicarbonate is an ideal choice for such a medium since it readily meets criteria 1, 2 and 4 above. Thus, it is relatively non-aggressive (Mohs hardness of about 2.5), is available in a variety of particle sizes, and is both water soluble and commonly utilized to treat sewage treatment facilities for the control of alkalinity and pH. The mild abrasive characteristics of sodium bicarbonate have previously been utilized, for example, in polishing media for teeth. See, for example, U.S. Pat. Nos. 3,882,638; 3,972,123; 4,174,571; 4,412,402; 4,214,871; 4,462,803; 4,482,322; 4,487,582; 4,492,575; 4,494,932, and 4,522,597.
The principal disadvantage attendant to the use of sodium bicarbonate as a blasting medium is its tendency to cake either by compaction or, more importantly, by exposure to high humidity conditions. This is particularly acute in commercial blasting operations, the compressed air streams for which are substantially saturated with moisture, i.e., have 90% or higher relative humidities, and contain oily contaminants from air compressors. In addition, commercially available sodium bicarbonate products have intrinsically poor flow characteristics due to their normal particle size distributions and crystal shapes.
The addition of flow aids to sodium bicarbonate to improve its flow and anti-caking properties is known. Thus, the blending of tricalcium phosphate (TCP) with sodium bicarbonate in baking formulas and dental air jet prophylaxis media has previously been proposed. The addition of such material substantially improves the flow and anti-caking characteristics of the bicarbonate. TCP-treated sodium bicarbonate is however, restricted to a 3 to 6 month shelf life under ambient conditions, since the TCP absorbs moisture until saturated, after which the product cakes.
It is, accordingly, among the objects of the present invention to provide bicarbonate-containing blasting media, and a process utilizing such media for removing coatings from sensitive metal and composite surfaces. Bicarbonate blasting media which may be so utilized are free flowing and have long storage lives under adverse commercial blasting conditions, and may be utilized as blasting media at high humidities and under a broad range of finely controlled, high flow rates and air pressures. Other objects and advantages of the invention will be apparent from the following description of preferred forms thereof.